1. Field of the Invention
The present invention relates to image forming apparatuses having a plurality of image forming portions, such as color printers and color copying machines. In particular, the present invention relates to an image forming apparatus which performs control of color shifting.
2. Description of the Related Art
In the related art, there has been image forming apparatuses which perform color shifting correction (e.g., see Japanese Patent Laid-open Nos. S64-6981, H5-241457, and 2004-145077).
In the image forming apparatus in accordance with Japanese Patent Laid-open No. S64-6981, a rewritable information writing portion is disposed respectively on each surface of a photosensitive drum and an intermediate transfer belt in the sub-scanning direction. The speed difference between the photosensitive drum and the intermediate transfer belt is detected by detecting information written in the information writing portions, and then, the rotation speed of the photosensitive drum is controlled in accordance with the speed difference. This is called a sampling correction method. This method is effective for shifting of the photosensitive drum fluctuating in a relatively long time due to temperature variation in the color image forming apparatus or due to external force applied to the apparatus.
However, in addition to the element which varies in a long time, color shifting includes an element which fluctuates in a short time to cause fluctuation of a rotation member. Such a color registration shifting element in a short time has not been a correction target of the sampling correction method in Japanese Patent Laid-open No. S64-6981. In other words, since correction is performed on the next image forming after detecting the color registration shifting of an image which was formed in the past, fluctuation in a short time cannot be corrected.
Japanese Patent Laid-open No. H5-241457 discloses the following technology. A visible image mark for position detection corresponding to original image information is formed at a first image forming portion. Subsequently, a detection signal is obtained by detecting the position detection mark which is transferred on the intermediate transfer belt. Writing operation timing of a second image forming portion is adjusted based on the detection signal.
In Japanese Patent Laid-open No. H5-241457, however, the position detection mark formed at the first image forming portion and transferred on the intermediate transfer belt thereafter is detected, and then, the wiring operation timing of the second image forming portion is adjusted based on the detection signal. Therefore, the fluctuation element generated within the period from the second image is formed at the second image forming portion until the second image is transferred to the intermediate transfer belt has not been detected and nor corrected.
In Japanese Patent Laid-open No. 2004-145077, the following technology is disclosed. A position information writing portion to write scales in the sub-scanning direction on respective surfaces of a photosensitive drum and an intermediate transfer belt is arranged. Speed difference between the photosensitive drum and the intermediate transfer belt is detected by detecting the scales written by the position information writing portion, and then, the rotation speed of the photosensitive drum is controlled in accordance with the speed difference. By writing a number of the information, detection of a fluctuation element in a short time has become possible and the color registration shifting has become possible to nearly be completely corrected.
However, when the color registration shifting is corrected by controlling the rotation speed of the photosensitive drum as disclosed in Japanese Patent Laid-open No. 2004-145077, such problems as described below occur.
For example, when the speed of the intermediate transfer belt is decreased, the rotation speed of the photosensitive drum is to be decreased. At that time, the position information writing portion writes the drum position information at constant time intervals on the photosensitive drum. Therefore, the space between the scales of the written drum position information becomes narrow.
Subsequently, even though the speed fluctuation of the intermediate transfer belt disappears once, the rotation speed of the photosensitive drum has to be decreased, in the case in which the speed of the intermediate transfer belt is decreased again when the drum position information arrives at a drum position reading portion. Accordingly, in this case, the space between the scales of the drum position information to be written on the photosensitive drum by the position information writing portion becomes narrower and narrower.
In this manner, the position correction control amount affects the movement correction control amount thereafter for each cycle of transferring from exposure. That is, the position correction control amount is accumulated when speed fluctuation continuously occurs. When the position correction amount is accumulated as described above, the position correction amount is oscillated with speed fluctuation of a specific frequency. There has been a problem that the position correction amount exceeds a position correction control range when the position correction amount is oscillated.
Measures for preventing position correction amount oscillation include one in which an image forming operation is completely stopped when the position control amount exceeds an established value. However, the complete stop of the image forming operation results in reduction in throughput of image forming.
A process of integrally moving an exposure portion and a photosensitive drum can also prevent the oscillation. However, since the mass of the exposure portion is large, high-speed fine move of the exposure portion integral with the photosensitive drum unavoidably results in the upsizing and higher cost of the movement mechanism.